Opposition leader Abhisit Vejjajiva said on Wednesday that Democrat MPs are fired up to challenge the government on the bill in the second reading debate, set for Thursday and Friday.
The party's main arguments are that there is no need for the Pheu Thai Party-led government to borrow money from outside to fund the projects, because the state coffers could fund them through the annual budget, and that many projects are vague in detail, or even have no details at all, and are wide open to corruption.
Mr Abhisit also expressed a new concern, that the government's tax receipts could miss the target and be unable to service the long-term loans, plus interest, for the projects.
More than 100 Democrat MPs are lined up to speak on the bill, raising the question whether they can all be accommodated in the two days allotted the debate.
Opposition chief whip Jurin Laksanavisit has urged more flexibility in the length of the debate, saying the House should extend the period to allow all MPs who want to speak the chance to grill the government.
Mr Jurin said the planned construction of the Pak Bara deep-seat port in Langu district of Satun was an example of the underlying flaws in the bill. No environmental or health impact assessments had been made, and yet it was still listed to benefit from the loans bill.
Democrat deputy leader Korn Chatikavanij said on Saturday that the party will take the loans legislation to the Constitution Court after its passes the third and final reading in the House, and will ask the court to kill the bill on the grounds it violates government fiscal discipline, which is a breach of the charter.
The government has already shown its intention to ram the bill through the House by deciding on Tuesday to cancel a planned mobile cabinet meeting in Lop Buri on Thursday and Friday - bringing complaints from local businesses there who had accepted bookings and planned for the meeting.
Pheu Thai has directed all MPs to be in the chamber for the debate, to ensure it is not adjourned for lack of a quorum -- which is lacking in the debating of many bills.
The decision to put the Lop Buri meeting on hold sparked speculation Prime Minister Yingluck Shinawatra would take the lead in defending the benefits of the bill in parliament. She is rarely present in the House for controversial issues.
Ms Yingluck has refused to answer questions on whether she will appear, saying only that the bill was scrutinised by a vetting committee.
She called for support from all MPs for the legislation, which she said would be a driver of the Thai economy in the long run.
Pheu Thai spokesman Prompong Nopparit dismissed concerns about corruption, saying all projects would be closely scrutinised to prevent the money leaking into other pockets.
The bill encountered no obstacle in passing the first reading in March, when the government used its majority with a vote of 282-152, with 21 members abstaining and seven deciding not to vote.
A big portion of the 2 trillion baht provided for in the loan bill will be used to improve transport, led by the construction of high-speed trains and dual tracks for ordinary trains.
The government has said borrowing the money with the authorisation of the bill is necessary to ensure the regular budget wlll not be affected. The government plans for a balanced budget in 2017.
The bill will clear the way for the Finance Ministry to obtain two trillion baht loans to overhaul infrastructure and end logistic bottlenecks, from 2014 to 2020. Improving infrastructure is a key policy of the Pheu Thai Party.
Its opponents point to the fact the loans will take about 50 years to pay off, placing a huge burden on future tax payers.